A proper analysis of the mechanisms involved in the control of breathing relies heavily on a complete understanding of the anatomical organization of the respiratory centers. The aim of this project is to study the nature and location of vagal afferent fibers in the brain stem respiratory centers. In the past, investigations of the sensory and motor terminations of the vagus nerve have posed great technical difficulty since these studies were based on degeneration techniques. The recent availability of sensitive neuroanatomical methods using the tetramethyl benzidine reaction for horseradish peroxidase (Kalia and Mesulam, 1978) has made it possible to study the afferent and efferent connections of peripheral nerves. In this project, we propose to use this technique on the vagus nerve with particular emphasis on the pulmonary components of the vagus. This is the first time this technique will be used to unravel the nature of pulmonary afferent termination in the respiratory centers. This study will link previous and ongoing research efforts by the principal investigator on the physiology of pulmonary reflexes and their central pathways. The principal investigator is currently involved in studying vagal afferent and central respiratory interactions and this proposed project will provide valuable neuroanatomical evidence that will be directly relevant to ongoing research in this laboratory. In essence, this project addresses the question: How do vagal afferents exert their influence on central respiratory mechanisms and where do these afferents terminate?